AFP was first detectable in maternal serum of pregnant monkeys (Macaca mulatta) at 6-8 weeks gestation (gestational length 24 weeks) using a sensitive (10 ng/ml) and specific radioimmunoassay. AFP concentrations rose rapidly and were maintained at 100-200 ng/ml for about 10 weeks. During the 3 weeks preceding parturition, serum AFP levels ranged from 19,000 to 27,000 ng/ml in contrast to maternal levels of less than 10 to 60 ng/ml. Maternal hypophysectomy produced a marked rise in maternal serum AFP at 6-7 days after surgery in 3 of 4 animals. Fetectomy at 10 weeks was followed by a similar rise and AFP was present in maternal serum at 5-6 weeks after removal of the fetus. Since the half-life of serum AFP was found to be 2 1/2-3 days, these results indicate retention or synthesis of AFP in an extra-fetal site as well as an effect of maternal stress on AFP levels.